In vitro digestibility of forages by coexisting deer species in Texas.
Percent in vitro digestible dry matter (IVDDM) of seven forages was estimated using rumen inocula from three fallow deer (Dama dama), two sika deer (Cervus nippon) and two white-tailed deer (Odocoileus virginianus) from the Edwards Plateau region of south-central Texas, 4-5 November 1987. Deer species source of inoculum affected IVDDM (P < 0.1) for four of seven forage species. Estimates of IVDDM for common curly mesquite (Hilaria belangeri), Texas wintergrass (Stipa leucotricha) and plateau live oak (Quercus virginiana fusiformis) were higher (P < 0.1) for sika deer than for fallow deer and white-tailed deer. This suggests that IVDDM results using rumen inoculum from a particular herbivore species should be applied with caution to other herbivore species. These results support the hypothesis that sika deer have a competitive advantage over white-tailed deer where these forages constitute a significant part of the diet, due to their ability to more completely digest these common range forages when highly digestible forages are limited.
The potential for financial returns from exotic big game production has increased interest in exotic species (Demarais et at. 1990). Although exotic ungulate populations have increased throughout North America in recent years, little is known of their impact on native flora and fauna. The Edwards Plateau region of south-central Texas has more exotic ungulates than any other place in North America, with 65 species and > 100,000 animals (Traweek 1989; Mungall & Sheffield 1994; Mungall 2000).
Research is needed to address concerns that exotic species are outcompeting native white-tailed deer (Feldhamer et al. 1978; Harmel 1980; Feldhamer & Armstrong 1993). The efficiency by which ungulates digest selected forages will influence competition of sympatric native and exotic ruminants for food resources. If an exotic ruminant's forage preferences overlap those of white-tailed deer, and the exotic is able to digest those commonly-preferred forages more efficiently, then the exotic will have a competitive advantage over white-tailed deer when preferred forages are limited.
Digestive efficiency of ruminants is commonly assessed by in vitro digestible dry matter (IVDDM). However, inoculum from a particular ruminant species may not accurately predict dry matter digestibility of other ruminant species (Blankenship et al. 1982; Van Hoven & Boomker 1983). The objectives of this study were to compare estimates of the proportion (%) of IVDDM for common range forages using inocula from exotic fallow and sika deer and native white-tailed deer to improve the understanding of competitive interactions among white-tailed deer and exotic deer and to document potential bias from source of inoculum when estimating IVDDM.
The study was conducted in south-central Texas on a 1,666 ha ranch in Bandera County. The vegetation was typical of the Edwards Plateau region and has been described as a brush discilmax (Buechner 1944). Dominant browse species in this region included ashe juniper (Juniperus ashei), plateau live oak and Vasey shin oak (Q. virginiana breviloba). Common grasses include Wright's three-awn (Aristida wrightii), grama grasses (Bouteloua sp.), hairy tridens (Erioneuron pilosum), common curly mesquite and Texas wintergrass. Forb diversity and availability were variable, depending on site, season and grazing history (Rollins & Bryant 1986).
The IVDDM of seven common forages was estimated using rumen inocula from three fallow deer, two sika deer and two white-tailed deer. All animals were female [greater than or equal to]2.5 years of age. Two plant species from grass, forb and browse forage classes and oak mast were collected on 23 October 1987 in Bandera County and adjacent Kerr County. The plant species were selected because they were available and potentially used by animals on the study area. The seven plant species selected were as follows: common curly mesquite, a warm-season grass; Texas wintergrass, a cool-season grass; yellow woodsorrel (Oxalis dillenii) and upright prairie-coneflower (Ratibida columaris) , warm-season forbs; live oak and ashe juniper leaves, evergreen browse species; and live oak acorns. Several plants of each species were randomly sampled and only current-year, leafy growth was collected. These samples were oven-dried at 60[degrees]C and ground through a 0.5 mm screen.
Inocula were collected from free-ranging animals which occupied native rangelands where the aforementioned common forages were endemic. No supplemental feeds were provided to animals on the study sites. Inocula were collected [less than or equal to]1 hr after animals were euthanized on 4 and 5 November 1987. Rumen fluid from each animal was strained through cheesecloth into an individual pre-warmed (39 [degrees]C) vacuum flask and saturated with [CO.sub.2] gas to maintain an anoxic environment (Spalinger 2000). The time lapse of about 3 hrs from collection to initiation of the digestion trials should not have affected the estimates of IVDDM (Schwartz & Nagy 1972). Analytical procedures followed those of Tilley & Terry (1963). Samples were run in duplicate during the two-stage digestion procedure with an incubation time of 48 hrs followed by pepsin digestion in the second stage.
Data were analyzed for each plant species using a one-way A NO VA with deer species as the treatment. Treatment differences were identified using Fisher's Protected Least Significant Difference (Kuehi 1994). Because of the small sample size within deer species (Tacha et al. 1982), [alpha] 0.1 was set for all statistical tests.
Rumen inoculum source affected IVDDM for curly mesquite, Texas wintergrass, live oak browse and live oak mast (Table 1). Sika deer inocula more completely digested (P [less than or equal to] 0.1) curly mesquite, Texas wintergrass, and live oak leaves compared to inocula from fallow deer and white-tailed deer. The IVDDM for live oak acorns was greater (P [less than or equal to] 0.1) for sika deer and white-tailed deer than for fallow deer. The forbs, yellow woodsorrel and upright prairie-coneflower, and the browse, ashe juniper, did not differ (P > 0.1) in IVDDM among deer species, although a trend for greater IVDDM using inocula from sika deer was apparent (Table 1).
Many researchers have extrapolated IVDDM results from inoculum from one herbivore species to another (Blair et al. 1977; Baker & Hansen 1985; Strey & Brown 1989). Palmer et al. (1976) compared cow in vitro and deer in vivo values and concluded that cow inocula provided an accurate estimate of deer food digestibility. Crawford & Hankinson (1984) found a high correlation between bovine IVDDM values and deer IVDDM values. Wheaton & Brown (1983) found digestive efficiency of sika and white-tailed deer in metabolic crates to be similar, but stated that the high-quality ration (21.2% crude protein, 15.9% crude fiber) may have affected the results. In accordance, results from this study indicated no differences in IVDDM values for forbs among deer species, which supports the hypothesis that these three species can digest high-quality forages with similar efficiency. However, others have shown differences in IVDDM values among ruminant species (Robbins et al. 1975; Blankenship et al. 1982; Van Hoven & Boomker 1983) a nd within species (Church & Peterson 1960; Nelson et al. 1972; Clary et al. 1988).
Van Hoven & Boomker (1983) concluded that in vitro digestibility of a given item is a function of inoculum source, time of year and feed selection by the donor animal. Differences in either composition or concentration of rumen protozoa could cause differential digestive efficiency among sympatric cervid species (Yokoyama & Johnson 1988). Dehority et al. (1999) found differences in the proportion of certain protozoa among sympatric cervid species as well as area-specific differences in the total protozoan concentration for cervids in the Edwards Plateau Region of Texas. Additionally, rumen microbial populations under the same dietary and environmental conditions vary from animal to animal, especially where nutritional stress may be present (Yokoyama & Johnson 1988).
Henke et al. (1988) found differences in the proportions of grass, forb and browse forage classes in the ruminoreticulum of fallow, sika and white-tailed deer. White-tailed deer selected primarily forbs (91 % of ruminoreticular contents), and fallow deer selected primarily grasses (94% of ruminoreticular contents). Sika deer were clearly the most generalist foragers, selecting 40% grasses, 48% forbs and 12% browse.
These differences in forage selection were logical considering differences in relative ruminoreticular capacity (Henke et al. 1988). The Sika deer's greater ruminoreticular capacity (Henke et al. 1988) should allow them to better digest more fibrous forages than white-tailed deer (Hanley 1982). Henke et al. (1988) concluded that sika and fallow deer were better able to digest grasses than white-tailed deer. However, despite differences in ruminoreticular capacity and the high proportion of grasses (94%) in the ruminoreticular contents of fallow deer reported by Henke et al. (1988), IVDDM estimates generated using fallow deer inocula did not differ from those generated using white-tailed deer inocula except for live oak mast.
The higher IVDDM values using sika deer inocula indicates that sika deer may be able to more completely digest certain forages compared to white-tailed deer and fallow deer. This could provide sika deer with a competitive advantage over white-tailed deer and fallow deer when forage resources are limited. Keiper (1985) established dietary overlap between free-ranging, sympatric populations of sika and white-tailed deer during autumn on Assateague Island, Virginia. In the same area, Feidhamer et al. (1978) and Feidhamer & Armstrong (1993) conjectured that sika deer competitively depressed white-tailed deer. In a Texas study (Harmel 1980), 6 sika deer increased to 62 while the coexisting white-tailed deer population declined to 0 in a 30-ha enclosure. Feldhamer & Armstrong (1993) surmised that the combination of relative rumen capacity and an opportunistic, generalist foraging strategy allow sika deer to displace white-tailed deer in sympatric populations. Davidson & Crow (1983) suggested differential susceptibi lities to infectious diseases and parasitism may affect competition between these two species as well.
The IVDDM estimates generated using fallow deer inocula did not differ from those generated using white-tailed deer inocula for the grass, forb, or browse forage classes, which indicates that fallow deer may not have a strong competitive advantage over white-tailed deer in sympatric populations. In a similar Texas study as described above, fallow and white-tailed deer were enclosed and monitored. In contrast to sika deer, fallow deer numbers declined as white-tailed deer numbers increased (Harmel 1992; Feldhamer & Armstrong 1993). However, fallow deer apparently displaced white-tailed deer after being introduced to Little St. Simon's Island, Georgia (Feldhamer & Armstrong 1993).
The results of this study suggest that IVDDM values are more accurate when derived from digestion trials using species-specific rumen inocula. Evidence suggests a myriad of conditions including the composition of rumen protozoa and microbes, location and previous forage selection interact to influence the digestive efficiency of a specific forage. Therefore, one must use caution when applying IVDDM results that were calculated using inocula from another ruminant species, or the same species collected in different seasons or locales.
Some exotic ruminants may digest forages more efficiently than white-tailed deer and may place them at a competitive advantage over the native ruminant in sympatric populations when forages are limited. Population size of exotic ruminants should be minimized when the management goal is reduction of potential competition with white-tailed deer.
Table 1 Percent in vitro digestible dry matter of seven common forages using rumen incoula from 3 cervid species sampled in Bandera Country, Texas, 4-5 November 1987. Fallow deer Forage Forage Category Category [F.sub.2,4] P x Grass Curly mesquite 15.5 0.013 18.5(A) (a) Texas wintergrass 5.3 0.075 23.3(A) Forb yellow woodsorrel 3.7 0.124 40.3 upright prairie- coneflower 1.9 0.265 58.4 Browse live oak 5.4 0.073 15.8(A) ashe juniper 0.8 0.500 22.8 Mast live oak acorns 27.5 0.005 14.0A Fallow Sika White-tailed deer deer deer Forage Category SE x SE x SE Grass 1.0 30.6(B) 0.9 15.1(A) 3.7 2.1 35.3(B) 2.3 22.5(A) 4.8 Forb 1.1 46.0 0.4 40.8 2.8 1.1 62.7 2.2 54.1 5.5 Browse 0.9 25.2(B) 3.5 15.7(A) 2.9 3.7 29.7 2.4 24.2 5.0 Mast 0.5 24.6(B) 0.2 25.0(B) 2.5 (a)Means is the same row with the same letter are not different (p > 0.1).
We thank D.A. Osborn, and The Texas Wild Game Cooperative for their assistance with data collection. We thank R. T. Ervin, I. M. Ortega, R. L. Preston, B. J. Rude and R. W. De Young for manuscript review. This study was funded by the Exotic Wildlife Association, Rob & Bessie Welder Wildlife Foundation, and Texas Tech University. This is contribution T-9-594 of the College of Agricultural Sciences, Texas Tech University, contribution 595 of the Rob & Bessie Welder Wildlife Foundation, and contribution WF189 of the Mississippi State University Forest and Wildlife Research Center.
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Department of Wildlife and Fisheries, Mississippi State University, Mail Stop 9690, Mississippi State. Mississippi 39762
Stephen Demarais, James J. Jackley, * Bronson K Strickland and Larry W. Varner **
* Range and Wildlife Management Department. Texas Tech University Lubbock, Texas 79409.
** Texas Agricultural Experimental Station. Texas A&M University Uvalde, Texas 78801.
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|Author:||Demarais, Stephen; Jackley, James J.; Strickland, Bronson K.; Varner, Larry W.|
|Publication:||The Texas Journal of Science|
|Date:||May 1, 2003|
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